Oscillation modulator



April 4, 1944- M. E. MoHR 2,345,712

OSCILLTION" MODULATOR Filed Feb. 18, 1945 ,L F/G/ LO/10 /24 /NVETOR M.E. MoH/P ALZOAZELW Patented Apr. 4,1944

OSCILLATION MODULATOR Milton E. Mohr, Summit, N. J., assignor to BellTelephone Laboratories, Incorporated, New York, N. Y., a corporation ofNew York Application February 18, 1943, Serial N0. 476,276

8 Claims.

This invention relates to an oscillation modulator and particularly toan amplitude modulator.

An object of the invention is to improve the over-al1 efliciency of amodulator without sacrince of quality and fidelity of the resultingsignals.

A more specic object of the invention is the effective amplitudemodulation of a carrier frequency with a minimum expenditure ofmodulating energy.

A feature of the invention resides in means for eifectively applyingmodulating voltages to an element of the oscillation circuit whichnormally would serve as an amplitude stabilizer of the oscillations.

There has previously been described (Patent 2,163,403, issued June 20,1939, to L. A. Meacham) a stabilized oscillator circuit wherein meansfor stabilizing the amplitude of the oscillations is included in one armof the stabilizing bridge circuit; this means in one embodiment is anonlinear resistance, e. g., a resistance having a positive temperaturecoefficient whereby the resistance value rises upon increase ofcurrentflow.

In accordance with a specific embodiment of the present invention, anon-linear amplitude stabilizer, in the form of a cold cathodegas-filled tube, contact rectifier, vacuum tube or the like, isconnected in one bridge arm of a bridge stabilized oscillator circuit.The modulating voltages are applied to this amplitude stabilizing.element whereby the characteristics of said element are varied at arate which is slow compared to that of the oscillator frequency; 'I'heresult is ellectlve amplitude modulation of the oscillations with aminimum expenditure of modulating energy.

(Cl. P19-171.5)

lReferring now to Fig. 1, there is illustrated a bridge type oscillatorcircuit including a vacuum tube I I which is preferably of the screengrid high ampliilcation type and which is provided with the usualelectrode energizing sources as illustrated. The lbridge networkcomprises four arms, two of which are formed by a closely coupledinductance coil, or transformer, I2 having a center tap which dividesthe coil into two equal sections. The positive terminal of battery I3 1sconnected to this center tap. The other two arms of the bridge comprise,respectively, resistance Il and a network which includes a tunedcircuit, comprising inductance I5 and capacitance I8 connected inparallel, and a third parallel arm comprising the secondary winding oftransformer Il, resistance 2I and gas-lilledtube 22. The value ofresistance 23 is high compared to that of resistance -2I.

A complete understanding of the operation of l the various arrangementscontemplated by the Fig. 3 illustrates a bridge stabilized oscillatorcircuit of the type in-which a piezoelectric crystall is included in onebridge arm and in which a biased rectifier is utilized as an amplitudestabilizer andmodulating means; and

Fig. 4 illustratesv a circuit similar generally to that of Fig. -2' bututilizing the plate characteristics of a pentode type vacuum tube forstabilization and modulation. l

The tuned circuit, comprising inductance I5.

and capacitance I6,controls the frequency of the oscillations producedby oscillator tube II in the general manner set forth in the Meachampatent referred to above while' gas-filled tube 22, acting in a mannerto .be described subsequently, controls the amplitude of theoscillations. The bridge is in balance when the combined impedance ofthe three parallel paths (tuned circuit and path through gas-filled tube22) is exactly equal to that of resistance Il. Feedback to the controlgrid of oscillator tube II is applied over feedback .path 24' and, asthis path is takenoif the junction point of resistance I4 and the tunedcircuit arm, it follows that any unbalance in the lbridge is translatedinto grid potential swing of oscillator tube Il.

oscillations produced by the circuit are applied to the load throughcoil 25 which is inductively related to coil I2. The modulating voltagesare applied, as described subsequently, to the circuit throughtransformer I 1. .A

Neglecting for the moment the modulating function of the circuit, let ussee lirst how gasillled tube 22 acts to stabilize the amplitude of theoscillationsv at the preselected value. At low amplitudes gas-filledtube 22 is essentially an open circuit being in deionized ornon-conductive condition. Under such conditions the impedance of thetuned circuit is substantially higher than the resistance value ofresistance I4; in other words, the bridge is unbalanced. When the plateof gas-filled tube 22. Tube 22 will ionize, or

' above.

break down at this point and become conductive. A conductive paththrough the secondary winding of transformer I1, resistance 2l and maingap of gas-filled tube 22 is established (resistance 23 being, as setforth above, of high value compared to resistance 2l) the result beingthat the combined impedance of the three-section arm (inductance I5,capacitance I6 and tube 22) is now brought down to a point only slightlyhigher thanthat of resistance I4 whereby the bridge is brought intoapproximate (but not exact) balance and the feedback over path 2l4 isreduced.

Under stable operating conditions gas-filled tube 22 will ionize, i. e.,become conductive, on the voltage peaks of each half cycle and willdeionize on each half cycle. The amplitude of the oscillations is,therefore, stabilized at the selected point as any increase of the peakvoltage across the tuned circuit above this point would be absorbed bythe path through tho ionized tube whileA a decrease of the peak voltagebelow the point would prevent ionization of tube 22, with its consequentstabilizing action, and the building up of the amplitude of the`oscillations would immediately recommence.

Considering now, in particular, the modulating action of the circuit,when no modulating voltages are being applied through transformer I1,gas-filled tube 22, operating as above described, will maintain the peakvoltage across the parallel combination of inductance I5 and capacitanceI6 at a value just slightly higher than the sustaining voltage of thetube. As modulating voltages are applied through transformer I1 they areapplied to gas-filled tube 22 in series with the voltage applied acrossthe tuned circuit by oscillator tube I I. The modulating voltages,therefore, effectively add to and subtract from the peak voltage appliedby oscillator tube I I, thereby causing gas-filled tube 22 to ionize(and restore) at a higher (or lower) point with respect to the output ofoscillator tube II than when subjected to the oscillator output alone.That is, the point at which the increase in amplitude of theoscillations is halted by ionization of gasfilled tube 22 is changedfrom moment to moment (increased or decreased) by addition (orsubtraction) of the modulating voltages to the peak voltage applied byoscillator tube II. This means, of course, that the alternating voltagesfed back overI path 24 to the control grid of tube II are varied in bothdirections from the stabilized value in accordance with the variationsin the modulating voltages; the net result is that the oscillationssupplied to the load by oscillator tube II through coils I2 and 25 aremodulated directly in accordance with the fluctuations in the modulatingvoltage applied through transformer I1. It will be understood that therate at which the ionization and deionization points of tube 22 arechanged is slow compared to the oscillator frequency.

It will be evident from the above description that, in accordance withthe novel arrangement contemplated by the invention, effectivemodulation may be achieved through relatively low power expenditure inthe modulating source.

Considering now Fig. 2, the circuit illustrated is similar generally to`that of Fig. 1 described Oscillations produced by oscillator tube 4Iare supplied to the load through coil 40 which is inductively related tothe primary winding of transformer 42. In the circuit of Fig. 2,however,

which includes inductance 44 and capacitance 45, thereby making possiblea grounded bias supply and low alternating current impedance to controlgrid of oscillator tube 4I over feedback path 46. The bridge includes asthe four arms, respectively, coil 48, coil 49, resistance and the threebranch arm, i. e., inductance 44, capacitance 45 and rectifier 41.Rectifier 41 in this instance acts as the stabilizing and modulatingelement. When no modulating voltages are supplied from the modulatingsource through transformer 5I, the alternating current peak voltagesupplied across the `parallel combination of inductance 44 andcapacitance 45 by oscillator tube 4I is held at a value just slightlygreater than the voltage of battery 52 and the amplitude of theoscillations produced by tube 4I is stabilized at this point. Now asmodulating voltages are applied through transformer 5I, they effectivelyadd to and subtract from, depending upon their instantaneous polarity,the voltage of battery 52, thereby causing corresponding changes in thevoltage supplied across inductance 44 and capacitance 45 therebyresulting in corresponding changes in the alternating voltage suppliedover feedback path 46, to the control grid of oscillator tube 4I. Theamplitude of the oscillations supplied to the load by oscillator tube 4Iis thereby modulated in accordance with the modulating voltage appliedthrough transformer 5I.

Considering now Fig. 3, the circuit illustrated is of a type inaccordance With which the oscillations supplied by oscillator tube 55 tothe load through output transformer 56 are normally stabilized by abridge which includes in three arms respective resista-hoes 51, 1I and12 and in the fourth arm a piezoelectric crystal 13. Alternating voltageis fed back to the bridge over feedback path 14. In this instancerectifier 15, biased by battery 16, serves as an amplitude stabilizerand modulating element. The path through rectier 15 to ground 18 is ineffect a shunt across bridge arm 51, decreasing the effective resistanceof that arm and bringing the bridge closer to an exact balance.

When no modulating voltages are being applied through transformer 11,the bridge, together with the biased rectifier, holds the oscillationsat the point of stabilization. When modulating volta ground 43 isconnected to the tuned circuit, 75

ages are applied to transformer 11l however, they effectively add to andsubtract from, depending upon their instantaneous polarity, the voltageof battery 16, thereby varying the bridge unbalance and causingcorresponding variations in the alternating'voltage supplied throughtransformer 8l to the control grid of oscillator tube 55. The amplitudeof the oscillations supplied to the load by oscillator tube 55 isthereby effectively modulated in accordance with the modulating voltageapplied through transformer 11.

Considering now Fig. 4, the circuit illustrated is similar generally tothat of Fig. 2 except for the fact that the plate characteristic of apentode type vacuum tube is utilized to stabilize and modulate theoscillator output. Oscillations produced by oscillator tube 84 aresupplied to the load through coil 83 which is inductively coupled totransformer 85. Pentode type vacuum tube 85 has its cathode-anode spacepath connected in parallel with the elements of the tuned circuit, i.e., inductance 81 and capacitance IUI, the alternating current impedanceof battery 88 being negligibly small, and this tube functions as anamplitude stabilizer and as a modulating element. The characteristics ofthe elements of the biasing voltage supplied to the control grid of tube86 whereby the plate currentrversus plate voltage relationship ofpentode tube 86 is varied in accordance With the variations of themodulating voltage. This variation of the characteristic of pentode tube86 in turn causes corresponding variations in the peak voltage acrossinductance 81 and capacitance |0| thereby resulting in a correspondingvariation in the alternating current feedback over path |02 to thecontrol grid of oscillator tube 84. The oscillations supplied to theload through transformer 85 are,l

therefore, modulated directly in accordance with the modulating voltagesapplied through transformer |03.

While certain specific embodiments of the invention have been selectedfor disclosure and detailed description, the invention is not, ofcourse, limited in its application to such embodiments. The embodimentsdisclosed should be taken as i1- lustrative of the invention and not asrestrictive thereof.

What is claimed is:

l. An amplitude modulator comprising a source4 of oscillations, a bridgeconnected in circuit with said source, a non-linear element connectedeffectively in shunt with elements of one arm of said bridge forstabilizing the amplitude of oscillations produced by said source, asource of modulating voltages and means for varying the characteristicsof said non-linear element in accordance with variations in themodulating voltages.

2. An amplitude modulator comprising a source of oscillations, a bridgeconnected in circuit with said source, a non-linear device connected inone arm of said bridge for limiting the amplitude of the oscillationsproduced by said source, a source of modulating voltages and means forvarying the characteristics ofy said non-linear device in accordancewith variations in the modulating voltages.

3. An amplitude modulator comprising a source of oscillations, a bridgeconnected in circuit with said source, a gas-filled discharge devicehaving a definite breakdown characteristic connected in one arm of saidbridge for stabilizing the amplitude of oscillations produced by saidsource at a value determined by said breakdown characteristic, a sourceof modulating voltages and means for varying the breakdowncharacteristic of said device in accordance with variations in themodulating voltages.

4. An amplitude modulator comprising a source of oscillations, a bridgeconnected in circuit with said source, one of the arms of said bridgecom- Drising a tuned circuit and a gas-filled discharge device having adefinite breakdown voltage characteristic connected in parallel, meansfor applying oscillations produced by said source across said tunedcircuit, said gas-illled discharge device being adapted to ionize whenthe peak voltage of the waves applied across said tuned circuit exceedsthe breakdown voltage characteristic thereof, ionization of said devicebeing effective to stabilize the amplitude of the oscillations at theapproximate value existent when said ionization took place, a source ofmodulating voltages, and means for varying the ionization point of saiddevice with respect to said peak voltages in accordance with thevariations in the modulating voltage.

5. An amplitude modulator comprising an oscillation device, a bridgeconnected in circuit with said oscillation device, one arm of saidbridge including means for stabilizing the frequency of oscillationsproduced by said device and means for stabilizing the amplitude of saidoscillations, said last mentioned means being connected in parallel withsaid first mentioned means and comprising a -gas-iilled discharge devicehaving a predetermined breakdown voltage characteristic, the breakdownvoltage characteristic of said device being a determining factor in theamplitude stabilization point, a source of modulating voltages and meansfor varying the breakdown point of said device with respect to theamplitude of said oscillations in accordance with variations in themodulating voltages.

6. An amplitude modulator comprising a source of oscillations, a bridgeconnected in circuit with said source, a rectier, a biasing battery forsaid rectifier, said rectifier and biasing battery being connected inseries with each other and in shunt with the elements of one arm of saidbridge, a source of modulating voltages and means for superimposingvoltages from said modulating source upon the voltage of said batterywhereby the biasing voltage applied to said rectifier is fluctuated inaccordance with the iiuctuations of the modulating voltages.

7. An amplitude modulator comprising a source of oscillations, a bridgeconnected in circuit with said source, a multi-electrode vacuum tube,means for connecting certain electrodes of said tube in one of the armsof said bridge whereby the plate current versus plate voltagecharacteristic of the tube is applied effectively in shunt with elementsof said one arm, a source of modulating voltages and means for varyingsaid plate current versus plate voltage characteristic in accordancewith variations in the modulating voltage.

8. An amplitude modulator comprising a source of oscillations, a bridgeconnected in circuit with said source, a vacuum tube having a cathode,anode, control grid, shield grid and suppressor grid, potential sourcesfor the electrodes of said tube, the cathode-anode space path of saidvacuum tube being so connected in one arm of said bridgey that the platecurrent versus plate voltage characteristic of the tube is appliedeiectively in shunt with elements of said one arm, a battery, means forapplying a biasing voltage from said battery to the control grid of saidtube for stabilization of said plate current versus plate voltagecharacteristic of the tube, a source of modulating voltages and meansfor superimposing voltages from said modulating source upon the voltageapplied to said control grid from said battery whereby the plate currentversus plate voltage characteristic is varied in accordance withvariations in the modulating voltages.

MILTON E. MOPH.

